The present invention relates to an opto-electronic measuring arrangement comprising an optical transmitter, an optical compensation transmitter and an optical receiver for measuring the transmission properties of a first transmission path between the transmitter and the receiver.
An opto-electronic measuring arrangement of this kind works according to the HALIOS principle developed by ELMOS AG. This measuring principle is known in the prior art and is described in the following documents, among others: U.S. Pat. No. 5,666,037; EP 0 706 648 B1; EP 1 671 160 B1; DE 100 01 955 A1; EP 2 418 512 A1.
The measuring arrangement, also designated hereafter as “sensor,” comprises an emitted light source also designated hereafter as “transmitter, emitter,” and a compensation light source, also designated hereafter as “compensator,” wherein each light source is energized, preferably in alternating manner, by its own current driver and emits light (usually in the IR spectrum) in the emitting phase or the compensation phase. For this purpose, a clock generator actuates the current driver with clock signals that are preferably phase-shifted through 180° with respect to one another. The frequency may be in the range from a few kHz to several tens of MHz.
An optical receiver receives a part of the light emitted by the two light sources with a photodiode, also designated hereafter as “PD,” and converts it into current alternating signals (AC), which—after separation of the DC and low-frequency signal components (generally originating from the ambient light)—are passed via a high-pass function (e.g., a capacitor) to a transimpedance amplifier (TIA). The transimpedance amplifier converts said current signals into voltage. These voltages are assigned to the emitting and compensation phases in a synchronous demodulator, again in alternating manner, and forwarded to a controller that is tasked with generating identical amplitudes for the two temporally consecutive signal components. To do this, the controller controls the amplitudes of the currents correspondingly through the compensator and the transmitter. Depending on the application, only the amplitude of the compensator current can be controlled with a constant transmitter current amplitude, or conversely only the amplitude of the transmitter current with a constant compensator current amplitude. The amplitudes of the compensator current are usually in the range of a very few mA. However, depending on the application, the emitter current amplitudes can be in the range from a few mA to several hundred mA.
The light radiated by the transmitter into the area around the sensor reaches the object being measured (detected) outside of the sensor. The object reflects a fraction of the light that reaches the object back to the photodiode of the sensor. The ratio derived from the received current in the photodiode and the transmitter (emitter) current used therefor is the optical coupling factor of the path from the transmitter via the measured object to the photodiode (emitter-measured object-photodiode path). The sensor determines the optical coupling factor and reproduces it in the sensor's controller adjustment signal.
The compensator (compensation light source) is constructed in such a manner that the light emitted thereby is not able to reach the measured object, but is directed to the photodiode inside the sensor instead. For this purpose, a waveguide or other optical guide is usually installed inside the sensor and guides the light from the compensator (directly) to the receiver, usually a photodiode. In practical application, the light component emitted by the compensator is set to a predefined dimension so that only a certain (usually smaller) fraction of the emitted light reaches the photodiode of the sensor during the compensation phase. The ratio between the current generated from this light fraction in the photodiode and the compensation current used therefor is the optical coupling factor of the compensator-photodiode path. This is constant, since the light emitted by the compensator essentially does not reach the measured object. Thus, the compensation signal represents an immutable parameter or a reference for the measurement.
Of course, other signals may also be used besides the temporally sequential signals described here, provided they are capable of generating feedback controlled compensation for the receiver output signal.